I'd put them on the side so that the cabinet doesn't roll all over the place when you're playing.

Actually, I'd build a dolly board (carpet covered plywood with castors) or get a folding hand truck. Either one is a few more bux but has many more uses other than just moving one speaker cabinet around...

I'm in the same situation, except I put my 15 on its side when I stack anything on top of it so casters on the bottom wouldn't be a problem (other than the rattle of them due to vibrations... tape I suppose).

I've definitely been thinking about building a dolly board or road case. Wouldn't have to screw anything into bottom of the cab that way, and can just get rid of it once it's in place.